As a method for protecting the surfaces of various materials such as a metal plate, a synthetic resin plate and a glass plate from contamination or damage, technologies of coating the surfaces with so-called protective films have been conventionally used. These protective films generally have a laminated structure that includes a substrate layer and an adhesive layer. The protective films are formed by applying a material for a pressure-sensitive adhesive layer on a substrate layer, or are produced by co-extruding a material for a pressure-sensitive adhesive layer with a material for a substrate layer. From the viewpoints of productivity and the production cost, the latter is industrially preferably employed.
Regarding the protective film, usually, a polyolefin-based polymer is used as a material of the substrate layer, and an acrylic block copolymer is used as a material of the pressure-sensitive adhesive layer. However, because the interlayer adhesive strength between a polyolefin-based polymer, which is a non-polar material, and an acrylic block copolymer, which is a polar material, is weak, in the case of employing a co-extrusion molding processing method, there has been a problem that delamination may easily occur. In order to solve this problem, for example, there have been suggested a method of providing an adhesive layer formed from an ethylene-vinyl acetate copolymer having a particular vinyl acetate concentration, between a substrate layer and a pressure-sensitive adhesive layer (see PTL 1); a method of providing an adhesive layer formed from a styrene-based resin only between a substrate layer and a pressure-sensitive adhesive layer (see PTL 2); a method of irradiating a laminate obtained by co-extrusion molding processing with ionizing radiation (see PTL 3); and a method of incorporating a polyolefin-based polymer into an acrylic block copolymer for a pressure-sensitive adhesive layer (see PTL 4).